1. Field of the Invention
The present invention relates generally to arc lamps, and more particularly, to arc lamps of the type which have short-arc gaps and integral, internal reflectors.
2. Description of the Prior Art
It is well known to utilize lamps having short-arc gaps to provide compact yet intense point sources of light. Such lamps are utilized for example, in medical and industrial endoscopes. Generally speaking, such lamps include a sealed chamber which contains a gas pressurized to several atmospheres, an anode and cathode mounted along the central axis of the concave chamber to define an arc gap, an integral concave reflector which serves to collimate light generated at the arc gap, and a window at the mouth of the chamber to permit external transmission of the collimated light from the lamp. In such prior art devices, current to the electrodes was direct current and, accordingly, electrons were emitted only from the cathode and collected only on the anode. To optimize the flow of DC current to create optical discharge, such anodes and cathodes at the arc gaps are shaped such that the tip of the cathode is pointed and the tip of the anode is relatively broader and blunted.
When utilizing direct current to power such prior short-arc lamps with integral internal reflectors, it is known to operate the lamps in a pulsed manner. During non-pulsed operation, a small current (known as the simmer current) is provided to the lamp, until such time as the lamp is pulsed; then the current is increased substantially (e.g. to about one-hundred amperes). In one known mode of operation, for example, the pulses are generated about one every 1.5 seconds and each pulse has a duration of about 100 milliseconds (i.e., one-tenth second), resulting in an energy flow across the short-arc gap of several hundred joules for the duration of the pulse. Typical voltages required for starting such lamps are approximately 12,000 volts.
One problem with short-arc lamps with internal reflectors and direct current drive, is that the anode and cathode electrodes physically deteriorate over time. Such deterioration can be caused by the high current inflow when starting the lamps or during pulsing. The deterioration normally manifests itself by sputtering of the tungsten material which forms the electrodes. One result of such deterioration is that the voltages required to start the lamp increase substantially. When the starting voltage becomes exceptionally high, the lamps must be replaced. In practice, such lamps are usually driven from an alternating current source via a rectifier which operates upon the alternating current to provide essentially direct current to the electrodes of the lamp.
It is also known in the art to utilize short-arc lamps which do not have integral internal reflectors. Such lamps are typically filled with xenon at pressures of several atmospheres. When the lamp is operated, gas pressure within the lamp may triple from the pressure when the lamp was cold. It is known to directly operate such lamps with alternating current fed to the lamp via a ballast and a current limiting device such as a choke coil or leakage transformer.